The present invention relates to the general field of electrically trace-heating metal pipes for transporting fluids, and it relates in particular to undersea pipes resting on the sea bottom and providing a connection from undersea wells for producing hydrocarbons, in particular oil and gas, to a surface installation for storage and offloading, e.g. a floating production unit.
In a single offshore hydrocarbon production field, it is common to operate a plurality of wells that may be spaced apart from one another by several kilometers, or even tens of kilometers. The fluids coming from those various wells need to be collected by undersea pipes made of metal (typically steel) laid on the sea bottom, and then transferred by bottom-to-surface connection pipes (“risers”) to a surface installation, e.g. a vessel or a collection point on land, that serves to collect the fluids in order to store them (and optionally process them).
The fluids coming from production wells tend to cool down very quickly while running along the numerous kilometers of undersea pipes. Unfortunately, if no provision is made for conserving some minimum threshold temperature inside such pipes, there is a major risk of molecules of gas, in particular of methane, that are contained in the fluids being transported combining at low temperature with molecules of water so as to form hydrate crystals. Such crystals can stick to the walls, where they agglomerate and contribute to forming plugs that are capable of blocking fluid flow inside pipes. Likewise, the oil solubility of high molecular weight compounds, such as paraffins or asphaltenes, decreases with lowering temperature, thereby giving rise to solid deposits that can also block flow.
One of the known solutions for attempting to remedy this problem consists in heating pipes by means of round or flat electric cables that are wound around steel pipes over their entire length in order to heat such pipes by the Joule effect. The electrical power that is supplied to the electric cables comes from an external electricity generator connected to the cables by an umbilical, or from an electricity generator that is integrated directly in the undersea pipes. That solution for electric heating is referred to as “heat-tracing” and serves to keep the fluids transported in the pipes at a temperature higher than a critical threshold over the entire path from the production well to the surface installation.
Typically, the electrical installation used for performing heat-tracing is made up of one or more three-phase electric circuits powered at different powers by one or more electrical transformers. Specifically, three-phase circuits are well adapted to delivering the electrical power needed for heating the pipe over its entire length with the lowest possible line losses.
In order to counter a failure, if any, of the electrical installation, leading to the heating of the undersea pipe being interrupted while it is in use, it is known to have recourse to redundant three-phase electric circuits in the installation, where such redundancy may be as much as 200% (i.e. two spare three-phase circuits are provided in addition to each three-phase circuit in use). When one of the three-phase circuits of the electrical installation fails, it is then replaced with one of the spare circuits. However, even though the failure of a three-phase circuit is generally due to the failure of only one of its electric cables, the circuit in question is scrapped entirely, including its electric cables that have not failed.